Nucleic Acid

Question 1

DNA

Answer 1

Function: hold or store genetic information; contains all the instructions for the growth and development of all organisms

Question 2

RNA

Answer 2

Function: transfer the genetic code in DNA out of the nucleus → carry it to the ribosomes in the cytoplasm

Question 3

Nucleotide structure

Answer 3

Each nucleotide is formed from:
-A pentose sugar (a sugar with 5 carbon atoms)
-A nitrogen-containing organic base
-A phosphate group

Question 4

DNA Nucleotide

Answer 4

The components of a DNA nucleotide are:

-A deoxyribose sugar with hydrogen at the 2’ position
-A phosphate group
-One of four nitrogenous bases - adenine (A), cytosine(C), guanine(G) or thymine(T)

Question 5

RNA Nucleotide

Answer 5

The components of a RNA nucleotide are:

-A ribose sugar with a hydroxyl (OH) group at the 2’ position
-A phosphate group
-One of four nitrogenous bases - adenine (A), Uracil(U), guanine(G) or cytosine(C)

Question 6

Purines

Answer 6

Adenine and guanine are purines - they have a double ring structure

Question 7

Pyrimidines

Answer 7

Cytosine, thymine & Uracil are pyrimidines - they have a single ring structure

Question 8

Phosphodiester bond

Answer 8

Nucleotides are joined via condensation reactions
occur between the phosphate group of one nucleotide and the pentose sugar of the next nucleotide

The reaction forms a phosphodiester (covalent) bond
Many nucleotides form a polynucleotide chain with a sugar-phosphate backbone

Question 9

Structure of DNA

Answer 9

Both antiparallel strands are held together by hydrogen bonding between complementary DNA base pairs

The purine adenine (A) always pairs with the pyrimidine thymine (T) – two hydrogen bonds
The purine guanine (G) always pairs with the pyrimidine cytosine (C) – three hydrogen bonds

Question 10

Structure of RNA

Answer 10

Made up of one polynucleotide strand
Much shorter compared to DNA
Examples include:
Messenger RNA (mRNA) - the transcript copy of a gene that encodes a specific polypeptide
Transfer RNA (tRNA)
Ribosomal RNA (rRNA)

Question 11

Ribosomes

Answer 11

The site of protein synthesis
Small organelles that are either free or attached to the rough endoplasmic reticulum
formed from rRNA and proteins
rRNA has enzymatic properties - catalyse the formation of peptide bonds

Question 12

What is semi-conservative replication?

Answer 12

1)DNA Helicase catalyses the unwinding of the double helix, by breaking the hydrogen bonds between complementary base pairs → strands separate
2)Free nucleotides are attracted to the exposed bases on each strand by base pairing
3)Nucleotides are then joined together by DNA Polymerase - catalyses the formation of phosphodiester bonds between adjacent nucleotides
4)Original & new strands are joined together through hydrogen bonding between base pairs

Question 13

How does DNA Polymerase work?

Answer 13

DNA polymerase cleaves (breaks off) the two extra phosphates and uses the energy released to create the phosphodiester bonds (between adjacent nucleotides)

BUT… Polymerase can only build the new strand in one direction (5’ to 3’ direction)

Question 14

Leading & lagging strands

Answer 14

The template strand that the DNA polymerase attaches to is known as the leading strand
The strand where DNA polymerase can synthesise the new strand continuously

The other template strand created is known as the lagging strand
DNA polymerase moves in the opposite direction
This strand is synthesised in short fragments (Okazaki fragments)

Question 15

DNA Ligase

Answer 15

hen joins these lagging strand segments together (phosphodiester bond formation) → forms a continuous strand

Question 16

Validating the Watson-Crick model of DNA replication

Answer 16

1) Bacteria with DNA containing heavier nitrogen
2)DNA settles at bottom of centrifuge tube
3)Bacteria with DNA containing only heavier nitrogen allowed to replicate in broth inside light nitrogen.
4)DNA settles in the middle as a mixture of both